This invention relates to electrostatographic reproduction machines, and more particularly to a hybrid development type electrostatographic reproduction machine having a system to detect a wrong-sign or wrong-polarity toner in a developer unit. Generally, the process of electrostatographic copying is executed by first using a corona generating or charging device to uniformly charge a photoreceptive member to a first polarity, and then exposing a light image of an original document, positioned in registration on a platen, onto the charged photoreceptive member. Exposing the charged photoreceptive member to a light image selectively discharges the photoconductive surface thereof in areas corresponding, for example, to non-image areas in the original document, while maintaining the charge (of the first polarity) on the image areas, thus creating an electrostatic latent image of the first polarity on the photoreceptive member.
The undischarged areas comprising the electrostatic latent image are subsequently developed with correct-sign or correct-polarity charged toner particles into a visible toner image. The sign or polarity of such correct-sign or correct-polarity toner, as is well known, is relatively opposite the first polarity of the latent image being developed. Ordinarily, toner particles are contained in the sump of a development apparatus where they are moved and mixed with carrier particles in order to triboelectrically charge the toner to the correct polarity. The toner image is thereafter transferred from the photoreceptive member onto a clean copy sheet on which the image is then fused or permanently affixed in order to provide a hard copy reproduction of the original document.
Unfortunately, the quality of the development step and that of the hard copy reproduction can be detrimentally affected by the effects of wrong-sign or wrong-polarity toner which forms and accumulates inside the sump of the development apparatus. Wrong-sign toner in the development sump of an electrostatographic reproduction machine is ordinarily the source of many machine performance failure modes, some of which can have catastrophic effects.
Wrong-sign toner is associated with a problem with excessive development of these wrong-sign toner particles in the background or non-image areas. This background development can be quite visible on the final copy sheet and is therefore a quite objectionable defect. The donor or developer roll which has predominantly correct sign charged toner on it might under certain conditions contain a small amount of oppositely charged carrier particles. The toner particles on this roll contact or develop both image and background areas on the photoreceptor of the machine. During such contact, wrong-sign toner particles on the donor or developer roll see each background area of the photoreceptor as a "development field" to which to transfer. This is because background areas are biased to repel correct-sign toner thus acting as a background cleaning field for toner of the correct sign or polarity. As such wrong-sign toner particles which have a polarity opposite to that of the correct-sign toner particles, obviously become attracted to these background areas, and so are transferred thus from and out of the developer housing.
It is therefore an object of the present invention to provide a method to detect detrimental amount of wrong-sign toner in development housings so to enable a process system to enter a correction mode to adjust for wrong signed toner. The correction mode could include lowering toner concentration; admixed cycle or entering in a purging mode.
The present invention utilizes a "hybrid" style development system, see for example Development Apparatus Having a Transport Roll Rotating at Least Twice the Surface Velocity of a Donor Roll", Jeffrey J. Folnins and Joseph Schram, D/89016, U.S. Pat. No. 5,063,875. In such a Hybrid system a magnetic brush dual component toner delivery system toner is applied to a "donor" or development roller according to the voltage difference between the DC bias on the donor and the loading "magnetic" roller. During normal operation the polarity and amplitude of the donor to magnetic roller bias difference is adjusted so that right signed toners on the magnetic roller are attracted and developed onto the surface of the donor roll from where they are transported into the region adjacent to the PR where they are then given the opportunity to develop any electrostatic latent images which might be present on the PR. During standby or non-imaging periods the donor to magnetic voltage difference is sometimes reversed to cause any toner on the donor roll to return back to the magnetic roller and be refreshed triboelectrically.